1. Field of the Invention
The present invention relates to an image processing system for use in such as an electrophotographic printer for example.
2. Description of the Prior Art
FIG. 8 is a schematic diagram showing the basic components of an electrophotographic printer. In FIG. 8, the electrophotographic printer comprises a photosensitive drum 1 supported to be rotatable in the direction of arrow "a", and around the photosensitive drum 1 is arranged electrophotographic equipment comprising an electrostatic charger 2, an LED print head 3 and a SELFOC lens array 4 constituting an image exposure system, a developing device 5, a print transfer device 6, a cleaning device 7 and a discharger 8. The electrophotographic printer also comprises transfer paper 9 which passes between the photosensitive drum 1 and the print transfer device 6 and is transported in the direction of arrow "b" by feed means (not shown), and a fixing device 10 where a toner image transferred from the photosensitive drum 1 to the transfer paper 9 is fixed to the transfer paper 9.
According to the above-mentioned constitution, while the photosensitive drum 1 is rotated in the arrow "a" direction, a latent image is formed on the surface of the photosensitive drum 1 which has been charged by the charger 2 through light focussed thereon by the image exposure system 3 and 4, and the latent image is made visible due to developing by the developing device 5, e.g. by the application of toner. After the visible image is transferred to the transfer paper 9 by the print transfer device 6, the surface of the photosensitive drum 1 is cleaned by the cleaning device 7 and subsequently discharged by the discharger 8, and then the charging is applied thereto again by the charger 2 and the above-mentioned process is repeated. On the other hand, the visible image transferred to the transfer paper 9 is fixed as a permanent image to the transfer paper 9 by the fixing device 10.
In the prior art, in the above-mentioned electrophotographic printer, a compact LED-head is adopted as part of a picture exposure system, but the LED chips corresponding to the individual pixels are several hundreds to several thousands in number per line and have different exposure amount characteristics.
Consequently, the LED head in the prior art transfers correction data to an LED driver IC so that uniform exposure can be performed.
FIG. 9 is a block diagram showing the constitution of an LED-head 20 in the prior art constructed so that uniform exposure can be performed. In FIG. 9, the LED-head 20 comprises a selector 11 for changing a picture signal clock (HCLK) into a picture signal transferring clock 11.sub.1 or a correction data transferring clock 11.sub.2 by a select signal (SELECT), and a shift register 12 for inputting picture signal data (HDATA) by line (l pixels serially in synchronization with the picture signal transferring clock 11.sub.1.
The LED-head 20 also comprises a latch circuit 13 for accepting and storing the contents of the shift register 12 in response to a latch signal (LATCH), AND gates 14.sub.1 -14.sub.l, for outputting the logical sum of the image signal output from the latch circuit 13 and strobe signals STB.sub.1 -STB.sub.m, and shift registers 15.sub.1 -15.sub.l for serially inputting correction data to correct each LED light quantity by P bits in synchronization with the correction data transferring clock 11.sub.2.
The LED-head 20 further comprises latch circuits 16.sub.1 -16.sub.l for accepting and storing the P-bit correction data from shift registers 15.sub.1 -15.sub.l in response to LATCH signals, current control circuits 17.sub.1 -17.sub.l for controlling current to actuate the LEDs in accordance with the content of the P-bit correction data, LED chips 18.sub.1 -18.sub.l arranged in a line, and a SELFOC lens array (SLA) 19 for focusing light from the LED chips 18.sub.1 -18.sub.l onto the photosensitive surface 1a of drum 1.
FIG. 10 shows a circuit example of a single current control circuit 17.sub.l. In FIG. 10, the control circuit 17.sub.l is constituted by transistors T.sub.1 -T.sub.p with each base connected to each bit terminal of the latch circuit 16 through a resistor RB.sub.1 -RB.sub.p, and a switch S.sub.l installed at a connection path to connect the emitter of each of the transistors T.sub.1 -T.sub.p respectively through resistors RE.sub.1 -RE.sub.p to the LED chip 18.sub.l and controlled by an output signal from AND gate 14.sub.l, so that current for the LED chip 18.sub.l is controlled in response to the contents of the P-bit correction signal latched to the latch circuit 16.sub.l. In practice, l is on the order of 2000-4000 pixels per line and the number of strobe signals m is on the order of 8, such that each strobe signal functions to activate 250-500 LEDs.
FIG. 11 is a block diagram of a picture processing circuit for controlling the LED-head 20 of the above-mentioned constitution. In FIG. 11, the picture processing circuit comprises a picture signal generator 21 for outputting a picture signal in response to line synchronous signal "a" from a CPU 28, a correction data generator 22 for outputting correction data in response to correction synchronous signal "b" from the CPU 28, a picture signal clock generator 23 for outputting a picture signal clock in response to the line synchronous signal "a" from the CPU 28, a correction clock generator 24 for outputting a correction clock in response to the correction synchronous signal "b" from the CPU 28, a selector 25.sub.1, for selecting one of the picture signal and the correction data in response to a select signal from the CPU 28, and a selector 25.sub.2 for selecting one of the picture signal clock and the correction clock in response to the select signal from CPU 28.
The picture processing circuit also comprises a rank correction output circuit 26 indicating a rank of the light quantity of the LED-head after being corrected by the correction data, a photosensitive body sensitivity output circuit 27 indicating the sensitivity of a photosensitive surface 1a formed on the photosensitive drum 1, and a crystal oscillator 29 for generating a master clock for the CPU 28. The rank of the LED-head refers to the overall level of brightness required to be produced by the LED-head according to the parameters of the specific apparatus in which it is installed and is determined before the LED-head is installed during manufacturing.
Next, operation will be described referring to the timing chart shown in FIG. 12. First, unevenness correction data to obtain even brightness between each pixel and obtained during unit adjustment of the LED-head 20 is stored in the correction data generator 22, and the rank, indicating the light quantity of the corrected result is stored in the rank correction output circuit 26. Also, data indicating the light sensitivity of the photosensitive surface 1a is stored in the photosensitive surface sensitivity output circuit 27.
The select signal "SELECT" from the CPU 28 is made "L" thereby indicating the correction mode, the correction synchronous signal "b" is outputted from the CPU 28 to thereby transfer correction data and the correction clock from the correction data generator 22 and the correction clock generator 24 to the LED-head 20 through the selectors 25.sub.1 and 25.sub.2. Also the latch signal "LATCH" for the correction data is simultaneously outputted from the CPU 28 to latch circuits 16.sub.1 -16.sub.l.
After the above-mentioned correction, the select signal is made "H" thereby indicating the printing mode. In the printing mode, the CPU 28 outputs the line synchronous signal "a" so that the picture signal from the picture signal generator 21 and the picture signal clock from picture signal clock generator 24 are transferred to the LED-head 20, and after completing a one-line transfer, the LATCH signal is outputted from CPU 28 to latch circuit 13.
Through strobe signals STB.sub.1 -STB.sub.m for the light emission of the LED-head 20, the ON-time of each LED 18 is controlled by the CPU 28 in accordance with data previously inputted from the rank correction output circuit 26 and the photosensitive surface sensitivity output circuit 27. Consequently, the strobe signals are outputted with the correct ON-time for the image to be printed according to the rank and the sensitivity of the particular printer, and the above-mentioned data transfer and output of the latch signal and the strobe signals are repeated for each line, and thereby the printing is performed.
Since the picture processing apparatus in the prior art is constituted as described above, uniform exposure can be performed by the LED-head having a number of light emitting diodes arranged linearly. However, regarding the printing content, for example, if the same exposure amount supplied to the pixels of a whole black portion is supplied to the pixel at an isolated point, a problem of generating unevenness of the printing density will occur in that the black density is low at the whole black portion and high at the isolated point.